Note: Descriptions are shown in the official language in which they were submitted.
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Title
Temperature Coeficient of Resistance ~odifiers
For Thick Film Resistors
Description
Technical Field
This invention relates to temperature
coefficient of resistance (TCR) modifiers for thick
fil~ resistors and more particularly ~o TCR modifiers
~or LaB6~based resistors fireable in nonoxidizing
atmosphere.
Background Art
Resistor and conductor compositions which
are applied to and fired on dielectric substances
(glass, glass-ceramlc, and ceramic) usually comprise
finely divided inorganic powders (~ ~, metal parti-
cles and binder particles) and are commonly applied
to substrates using so-called "thick film" techniques,
as a dispersion of these inorganic powders in an
inert li~uid medium or vehicle. Upon firing or sin-
tering of the film, the metallic component of thecomposition provides the functional (resistive or con-
ductive) utility, while the inorganic binder (e.q.,
; glass, crystalline oxides such as Bi203, etc.) bonds
the metal particles to one another and to the sub-
strate. Thick film techniques are contrasted with
. . :, . ~, . ~
. .
thin film techniques which involve deposition of
particles by evaporation or sputtering. Thick film
techniques are discussed in "Handbook o~ Materials and
Processes Eor Electronics", C. A. Harper, Editor,
McGraw-Hill, N.Y., 1970, Chapter 12.
One of the important characteris-tics of
electrical resistors is their TCR. Many electrical
resistors of the prior ar-t, based on, e.~., precious
metals, possess certain undesirable properties such as
high TCR. See, e.~., D'Andrea, United States Patent
2,924,540, issued February 9, 1960 and Dumesnil, United
States Patent 3,052,573, issued September 4, 1962.
More recently certain electrically conductive
materials were found whose resistivi~y is virtually
independent of temperature over a wide range of
temperatures. See, e.~., Bouchard, United States Patent
3,583,931, issued June 8, 1971 and Hoffman, United
States Patent 3,553,109, issued January 5, 1971.
Resistors based on the pyrochlore-related materials
described in the above two patents, however, were found
to be incompatible with copper conductors under certain
conditions.
Resistors compatible with copper conductors
have been found recently and are described in copending
25 Canadian Patent Application Serial ~o. 333,754 to
P. C. Donohue, filed 1979 August 14, concurrently with
the present application. Although these resistors have
relatively low TCR values, there are many applications
which require even lower TCR's.
Disclosure of the Invention
The TCR modifiers of this invention comprise
1-20 parts by weight of the solids content of the
resistor compositions. The solids content of
~l~3~ 2
the resistor compositions of this inventlon consists
essentially of a conductive material such as
lanthanum hexaboride (LaB6), yttrium hexaboride -~-
(YB6~, the rare earth hexaborides, calcium hexa-
boride (CaB6), barium hexaboride (BaB6), strontium
hexaboride (Sr~6) or mixtures thereor; glass;
and TCR modifiers. The weight ratio of hexa-
boride to glass is from 10:90 to 95:5.
The resistor composition comprises the
above solids content dispersed in a vehicle which can
be a solution of an organic polymeric material in
a solvent, is compatible with copper conductors,
and is fireable in a nonoxidizing atmosphere. The
TCR modifiers of this invention are semiconductors
and include TiO, Si, Ge, and C.
~escription of the Invention
.
The TCR modifiers of this invention comprise
1-2Q parts by weight based on the solids content
of the resistor composition of this invention,
perferably 2.5~15 parts, and more preferably 2.5 lO
parts. Another preferred range is 1-4 part by
weight.
In general, thick film resistor compositions
have, unless modified, relatively large positive TCR
values at the low resistivity range of the resistor
films and negative values in the high resistivity
range. Such a behavior is thought to occur
because of the domination of the metallic functional
phase in the low resistivity range while, in the
high resistivity range, the semiconduc~ing
characteristic of the functional phase-glass
junctions dominate.
To be useful TCR modifiers for resistor
composition of the instant invention, the TCR
modifiers of this invention ha~e to be substan-
tially nonreactive with the hexaboride functional
~3~2
phase and nonreducible by the hexaboride.
The TCR modifiers meeting the above
criteria are TiO, its higher oxidation state
precursors Ti305 and Ti203, NbO, TaO, C, Si,
Ge, SiC, compounds based on groups III A and V A
of the periodic chart of the elements such as
gallium arsenide, GaAs, compounds based on
Groups II B and VI A of the periodic chart
such as cadmium teliuride, CdTe, rare earth
nitrides, certain Group III B nitrides such
as LaN or mixtures thereof.
The present inventive selection of TCR
modifiers for the resistor compositions described
herein is based on a combination of considerations.
Although a reaction scheme according to the
equation below can be visualized, such a reaction
is ~hought to occur to a limited degree only, if at
all, and, therefore, 5i, etc. are suitable as TCR
modifiers in the present invention: LaB6 +x~i
LaSix+6B.
Si, C, Ge are also resistant to reduction
by the hexaborides; Si~4, CH4, GeH4 are thought to be
unlikely end products in presence of hexaborides.
It is known that hexaborides such as
LaB6 are strong reducing agents; their reaction
with m~tal oxides (MO) is shown below:
LaB6 + LO.5 MO ~ 0.5 La203 + 3 B203 + 10.5 M
Based on thermodynamic calculations and approxl-
mations, the Gibbs free energy of formation, ~F,
will be zero (at 900C, the approximate average
temperature at which the thick film resistor films
are formed from the compositions of this invention)
when ~F (M~O) is approximately -80.8 kcal/mole.
This means that only those semiconductor oxides
will be satisfactory TCR modifiers of this invention --
which have ~F(per each M-O bond in the molecule)
less than -80.8 kcal/mole. Under these circumstances
there can be no reaction between LaB6 and the TCR
modifier oxides. For example, ~F~ (at 90~ ~) of TiO
is ~96.65 kcal/mole, theoretically showing it to be
a satisfactory TCR modifier of this invention.
Although ~F values (at 1200K) for Ti203
and Ti305 are -94.8 and -92.0 kcal/Ti-O bond,
respectively, and therefore no reduction by LaB6
is expected, potential stepwise reduction during
firing would lead to TiO, a good TCR modifier.
(Thè thermodynamic data utilized herein are based on
tabulations in Bulletin 542, Bureau of Mines,
"Con`tributions to the Data on The~retical Metallurgy;
XII. Heats and Free Energies of Formation of
Inorganic Oxides" by J. P. Coughlin, 1954. aFLaB6
20 was estimated to be approximately 10% less than
its reported ~H of -30.7 kcal/mole; see Chem. Abstr.,
70:61844 v.)
Among the TCR modifiers disclosed above, TiO,
Ge, Si, and C(in crystalline graphite form) are
preferred. Their utilization leads to thick film
resistor films of good electrical properties,
stability, and adhesion to substrate.
The effectiveness of these TCR modifiers can
only be judged in its proper perspective if, in
addition to lowering the TCR value of a resistor to
which it had been added toward the ideal value of
zero, the modified resistor is to the left of the so-
called "universal" curve.
It has been found ~hat for a given thick
film resistor system (same conductox, similar types
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and amounts of glasses of similar particle size), the
plot of TCR values versus the log of resistivity (ohms/
square/thickness) is a curve approaching a straight
line having a negative slope. The line passes from
the positive TCR range into the negative TCR range.
Effective TCR modifiers afford thic]c film resistors
having TCR/resistivity data points to the left of the
"universal" curve corresponding to the unmodified
resistor system.
Different thick film resistor systems have
different "universal" curves and, within each system,
the TCR values diminish as the resistivity increases.
Since different final applications can tolerate
different maximum TCR values, the amount and type of
TCR modifier can depend on the particular utilization
of the resistor system and on whether the lower or
higher resistivity range is necessary. Although no
absolute maximum acceptable TCR value is generally
known, it is thought that resistors having a TCR
value outside of the + 250 ppm/C range are
unacceptable.
The remainder of the solids content of the
resistor composition of this invention, 99-80 parts
by weight, comprises a conductive material and glass.
An especially preferred conductive material-
glass combination is one in which the conductor is a
hexaboride such as lanthanum hexaboride (LaB6),
yttrium hexaboride (YB6), the rare earth hexaborides,
calcium hexaboride (CaB6), barium hexaboride (BaB6),
strontium hexaboride (SrB6) or mixtures thereof and
the glass is nonreducible glass. Such a composition
is described in a copending Canadian Patent Applica-
tion Serial No. 333,754 to P. C. Donohue, filed 1979
August 14 concurrently with the instant application.
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Other commonly known glasses can also be
utilized, such conventional glasses contain, among
other constituents, ~IgO, CaO, SrO, BaO, ZrO2, l~lnO,
Fe203, CoO, ZnO, B203. ~hese glasses are prepared
5 by conventional glass-making techniques, by mixing the
desired components (or precursors thereof, ~
H3B03 for B203) in the desired proportions and
heating the mixture to orm a melt. As is well
known in the art, heating is conducted to a peak
10 temperature and for a time such that the melt becomes
entirely li~uid, yet gaseous evolution has ceased.
In this work the peak temperature is in the range
llOO-l500~C, usually 1200-1400 ~. The melt is then
fritted (particles are made) by cooling the melt,
15 typically by pouring onto a cold belt or into cold
running water. Particle size reduction can then be
accomplished by milling as desired.
The relative amounts of conductive hexa-
boride and glass to be utilized can ~ary depending
20 on the particular application of the final resistor
film: 10-95 parts by weight, based on the weight of
hexaboride plus glass, of hexaboride and 90-5 parts
by weight of glass, preferably 15-50 parts and 85-50
parts, respectively, are useful and preferred ranges.
The solids content of the resistor
composition of this invention is dispersed in a
vehicle.
Any inert liquid can be used as the vehicle.
Water or any one of various organic liquids, with or
30 without thickening and/or stabilizing agents and/or
other common additives, can be used as the vehicle.
Exemplary of the organic liquids which can be used
are the aliphatic alcohols; esters of such alcohols,
for example, the acetates and propionates; terpenes
35 such as pine oil, terpineol and the like; solutions
of resins such as the polymethacrylates of lower
,~ , .
' ; ' ~
~ ~ 3~2
alcohols, or solutions of ethyl cellulose, in
solvents such as pine oil and the monobutyl ether
of ethylene glycol monoacetate. The vehicle can
contain or be composed of volatile liquids to
promote ~ast setting after application to the
substrate.
One particularly preferred vehicle, based
on copolymers of ethylene and vinyl acetate, is
described in a copending Canadian Patent Application
Serial No. 333,756 to D. H. Scheibex, filed 1979
August 14 concurrently with the instant application.
The ratio of inert liquid vehicle to solids
in the resistor compositions of this invention can
vary considerably and depends upon the manner in
which the dispersion of resistor composition in
vehicle is to be applied to the kind of vehicle used.
Generally, from 0.5 to 20 parts by weight of solids
per part by weight of vehicle can be used to produce
a dispersion of the desired consistency. Preferred
dispersions contain 10-35 parts by weight of vehicle
and 90-65 parts by weight of solids.
The resistor compositions are prepared from
the solids and vehicles by mechanical mixing. The
resistor compositions of the present invention are
printed as a film onto ceramic, alumina or other
dielectric substrates in the conventional manner.
Generally, screen stenciling techniques are p~efer-
ably employed. The resulting printed patterns are
generally allowed to level out, dried at elevated
temperatures such as at 120C for approximately 10
minutes, and fired in nonoxidizing atmosphere in a
belt furnace at a peak temperature of approximately
910C.
Preferably, a nitrogen atmosphere is employed
in the furnace but other nonoxidizing yases such as
.
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,~;
3~
hydrogen or a mixture of hydrogen and carbon monoxide
can also be used. Also, small quantities of oxygen can
also be present during ~he firing without adversely
aEecting the final resistor properties. It is believed
that a maximum o~ approximately 100 parts per million of
oxygen i5 permissible; above this level, oxygen appears
to have a TCR modiEier effect. It is thouyht that
oxygen induces metal oxide formation at the conductor-
conductor interfaces imparting semiconducting character-
istics to regions of -the compositions and thereby acting
as a negative TCR modiier. It is however, possible
that depending on the exact nature of hexaboride, glass,
and vehicle, higher levels of oxygen can be tolerated
without any adverse effect. The preferred range of
oxygen content in the nitrogen atmosphere is 3-30 ppm.
Resistance measurements can be carried out in
a two-probe procedure utilizing a digital ohmmeter.
TCR measurements are carried out by measuring
resistivities (RE: RRT) at elevated temperatures (TE)
between 125C and 150C and at room temperature (TRT).
TCR is calculated from the following formula; in units
of ppm/C:
TCR = RE-- - R-RT
RRT(TE ~ TRT)
Often, for better comparison, the measured resistivities
are normalized to a uniform thickness.
Filrn-thickness (and, also, surace roughness
of the films) is measured by a co~nercial instrument,
the GOULD* surfanalyzer, which records surface profile,
thickness values, and surface roughness.
In the Examples below, illustrating the
invention, all parts are by weight unless otherwise
indicated.
* denotes trade mark
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Examples
Examples 1-14
A series of resistor compositions is prepared
containing incremental amounts of Tio and varying ratios
of LaB6 to glass. The solids content is LaB6,
vibratorily mi~led for 7 hours (vibratory milling is
carried out in an aqueous medium by placing inorganic
powders and steel balls into a container which is then
vibrated for a specified length of time) to a surface
area of g.2 m2/g; TiO2, vibratorily milled for 16
hours to a surface area of 4.4 m2/g, and a glass
having a surface area of 4.2 m2/g.
The glass is a nonreducible glass prepared
from the following constituents (mole~): B203(25.38),
SiO25~6.70), A1203(12~69), CaO(12.69), ZrO2(2.03), and
Tio2(0.5Q7) and is ully described in the aformentioned
Canadian Patent Application Serial ~o. 333,754 to
P. C. Donohue.
The resistor composition is prepared into a
paste form by dispersing LaB6 and glass in the vehicle
by HOOVER* milling, followed by the addition of
different amounts of TiO to the dispersions containing
varying ratios of LaB6 and the glass.
The vehicle is a solution of an ethylene-vinyl
acetate (45/55 by weight) polymer, having a melt flow
rate of 1.0, dissolved in hexyl CARBITOL* to provide a
14~ by weiyht solution. The vehicle comprises 30% by
weight of the total resistor composition. This vehicle
is described in the aforementioned Canadian Patent
Application Serial ~o. 333,756 to D. H. Scheiber.
The following is the tabulation of the
quatities (g.) utilized in preparing the vari~us
samples. (The numbers in parenthesis indicate the
weight ratios of the components oE the solids content).
~ denotes trade mark
. ~'`, '
~3~72~Z
11
ExampleLaB6 glass TiO vehicle
pol~mer solvent
1.8(60) 1.2(40) 0.18 l.ll
2 1.5(50) 1.5(50) - 0.18 1.11
3 1.2(40) 1.8(60) - 0.18 1.11
4 0.9(30) 2.1(70) - 0.18 1.11
0.6(20) ~.4~80) - 0.18 1.11
6 1.8(60) 0.9(30) 0.3(10) 0.18 l.ll
7 L.5(50) 0.9(30) 0.6(20) 0.18 l.ll
8 1.5(50) 0.6(20) 0.9(30) 0.18 1.11
9 9.6(32) 20.4(68) - 1.8 11.1
11.7(39) 18.3(61) - 1.8 11.1
11 18.0(60) 12.0(40) - 1.8 11.1
12 9.6(31,1) 20.4(66) 0.9(2.9) 1.8 ll.l
15 13 11.7~37.1) 10.3(~8.1) 1.5(4.8) 1.8 11.1
14 18.0~6.1) 12.0(37.4) 2.1(5.5) 1.8 11~1
The resistor compositions prepared above
are screen printed over prefired copper electrodes
usinq 3 25 mesh screens, allowed to level
~or 10 minutes, dried at 12û~ for 10 minutes, and
fired in a belt furnace in a nitrogen atmosphere
containing 25-30 ppm of oxygen in the burn-out
zone and 3-10 ppm of oxygen in the firing zone.
The total firing cycle is 56 minutes, reaching a
peak temperature of 910C for 6-8 minutes, at a rate
of temperature rise and fall of approximately ao-
100 ~/minute.
The resistivity values are measured as
described above and the TCR values are calculated
using the equation shown above. Values are
tabulated below:
11
,
., ~
,. .
,
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- ~L3~
12
Exam~le Resistivity TCR
[ohm/square/0.5mil(0.0127mm)] (pp~/~C)
1 10.6 430
2 17.9 470
3 79.1 300
4 18~0 96
~
6 7.6 0
7 10.0 -2~0
3 7.8 -4~0
9 775 210
82.8 340
11 11.7 380
1? 557 118
13 108 114
14 13.8 159
As can be seen from these data, the resistor
compositions containing no TiO TCR modifier fall on
the "universal" TCR versus R curve within experimental
error. (It needs to be recognized that, in working
with small quantities of materials, there are
introduced relatively large experimental variations
especially with respect to sample uniformity and
reproducibility of data.)
Examples 6~7 and 12-14 show the beneficial
effects of TCR modification by the claimed TCR
modifier and show that these resistor compositions
are moved off of the "universal" curve. Example 8
affords a thick film resistor having an unacceptably
large nega~ive TCR value.
Example 15
A resistor composition i~ prepared from
haB6(0.3g.), a glass [prepared from SrO(40 mole%),
A1203(20 mole%~, B203(40 mole~) as per copending
12
,:
~ ~ 3~
Canadian Patent Application Serial No. 333,754 to
P. C. Donohue, filed 1979 August 14, 0.2g.],
Si(0.02g.), and a vehicle of 10% by weight of ethyl
cellulose [havin~ a vi.scosity of 22 centipoises in
a 5% by wei~ht solution in cl solvent of toluene/
ethanol//80/20 (by weight), 47.5-49.0~ ethoxyl
content, and 2.4-2.53 ratio of ethoxy gxoups to
anhydro-glucose units] in ~-terpeneol, in a manner
described above. The vehicle is present in a
quantity sufficient to prepare a printable paste.
Thick film resistors are prepared as in
Examples 1-14, having an average TCR value of -107.
Examples 16-19
A series of resistor compositions, containing
varying levels of Ge, is prepared from LaB6, a glass
[prepared from B2O3(33.6 mole~), SiO2(44.7 mole%),
A12O3(6.7 mole%), CaO(15 mole%), as per the copending
patent application referred to in Example 15 above],
and a vehicle, in a manner described above. Thick
films are prepared as in Examples 1-14.
The following is the tabulation of the
quantities (g.) utilized in preparing the various
samples. (The numbers in parenthesis indicate the
weight ratios of the components of the solids
content.)
,: :
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~372~Z
14 (1)
Example La~ Glass Ge Vehicle
16 3.4(68) 1.~(32) 0 2.82
17 3.~(68) 1.6(32) 0.25(5)2.82 -
18 3.4(68) 1.6(32) 0.50(10)2.82
19 3.~(68) 1.~(32) 1.0 (70)2.82
The resistivity values are measured as
descrihed above and the TCR values are calculated
using the equation shown above. Values are
-tabulated below:
10 Exam~le Resistivity TCR
..
[ohm/square/0.5mil(0.0127mm)1 (ppmC)
16 14 500
17 14 300
18 17 280
lS 19 28 190
- As can be seen from these data, Ge is an
effective TCR modifier.
Examples 20-25
A series of resistor compositions, containing
various levels of graphite (crystalline), is
prepared from LaB6, glass (same as in Examples 16-19),
and a vehicle, in a manner described above. Thick
film resistors are prepared as in Examples 1-14.
(~) The vehicle contains 1.25 g. of a commercial
hydroxy-terminated polybutadienel 0.67 g. of a
25% by weight solution of polyiso-butyl meth-
acrylate (inherent viscosity = a . 7 deciliter/gram,
measured at 20C in a solution of 0.25 g. of
polymer in 50 ml. of chloroform) in 2,2,4-tri-
methylpentadiol-1,3-monoisobutyrate, and 0.90 g.
more of this same solvent.
-- 14
: ' - ' ;
~3~
The following is the tabulation of the
quantlties (g.) utilized in preparing the various ~-
samples. (The numbers in parentheses indicate
5 the weight ratios of the components of the solids
content.)
ExamE~LaB6 Glass Çraphite Vehicle
4.2(84)0.8(16) 0 282
21 4.2(84)0.8(16) 0.05(1)282
10 22 4.2(84)0.8(16) 0.25(5)282
23 4.2(84)0.8(16) 0.50(10) 232
24 4.2(84)0.8(16) 1.0 (20) 282
4.2(84)0.8(16) 1.5 (30) 282
The resistivity values are measured as
15 described above and the TCR values are calculated
using the e~uation shown above. Values are
tabulated below:
Example Resistivity TCR
~ohm/square/0.5mil(0.0127mm)] (ppm ~)
20 20 5 600
21 8 440
22 8 360
23 8 200
~4 15 60
25 25 44 -360
As can be seen from these data, Examples
23-24 exemplify useful levels of graphite as TC~
modifier for this particle combination of hexaboride,
glass, and vehicle.
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